In some type of air conditioners, an outdoor unit and an indoor unit are connected to each other by three lines: a power supply line, a signal line for signal transmission, and a common line shared by an alternating current (AC) transmission and signal transmission. Examples of such air conditioners include an air conditioner in which power supply to an outdoor unit control circuit is shut off during standby in order to reduce standby power consumption, as described in, for example, Patent Document 1.
Specifically, in the air conditioner of Patent Document 1, an outdoor unit includes an outdoor relay for opening/closing connection between an outdoor unit controller and a main power supply and a relay driver for driving the outdoor relay, and an indoor unit includes a driving power supply for supplying driving power to the relay driver and an indoor relay for opening/closing connection between the driving power supply and the relay driver. In a shift to a standby mode, the indoor relay is switched, thereby supplying driving power from the driving power supply to the relay driver through the signal line. Once the driving power has been supplied to the relay driver, the relay driver switches the outdoor relay to sever the connection between the main power supply and an indoor unit controller. In this manner, power supply to the indoor unit controller is shut off.